1. Field of the Invention
The present invention relates to an optical recording medium for recording and reproducing information thereon by radiation of a laser light or the like; the invention further relates to a substrate for such an optical recording medium, a master disc, a master disc recording apparatus, and a signal generating apparatus used in the same.
2. Related Art of the Invention
Optical recording media are attracting attention as large capacity, high density memories, and the development of rewritable media called the erasable type has been proceeding. One of the erasable optical recording media is the phase change optical disc which has, on a disc-shaped transparent substrate, a recording layer made of a thin film that exhibits a phase change between an amorphous state and a crystalline state, and on which information is recorded and recorded information erased utilizing the heat energy generated by radiation of a laser light.
The production process for the phase change optical disc comprises (1) a film deposition step in which a phase change type recording layer is formed by sputtering or other technique on a transparent substrate having on its surface a guide groove formed from raised and recessed portions, (2) a laminating step in which a protective plate for protecting the recording thin film is laminated on the recording layer, and (3) an initializing step in which initialization is performed to change the state of the recording layer from the amorphous to the crystalline state. As phase change materials for the recording layer, alloy films composed principally of Ge, Sb, Te, In, etc., for example, a GeSbTe alloy, are known in the art.
Generally, information is recorded by forming a mark by locally amorphizing the recording layer, and erasure is done by crystallizing this amorphous mark. Amorphization is accomplished by heating the recording layer above its melting point and then cooling it at a rate faster than a prescribed value. On the other hand, crystallization is done by heating the recording layer to a temperature equal to or higher than its crystallizing temperature but equal to or lower than its melting point.
In general, a spiral or concentric guide grooves for guiding a laser beam during recording and reproduction, and address information (hereinafter sometimes called the address for short) consisting of address pits formed from raised and recessed portions indicating a position on the medium, are provided in advance on the substrate.
Further, the address information is formed in the arrangement called the ZCAV or ZCLV format, and each track is divided into a plurality of recording sectors by address sections formed from address pits. The number of sectors per revolution differs from zone to zone arranged in the radial direction. More specifically, the number of sectors gradually increases toward the outer diameter of the medium. In the same zone, the number of sectors per revolution is the same. Further, the number of zones depends on sector length, and decreases with increasing sector length.
With increasing processing capabilities of information apparatuses in recent years, the amount of information they handle have been increasing. There is thus a need for recording media capable of recording and reproducing larger amounts of information. As a means of increasing the capacity, DVD-RAM, for example, employs a method that uses both the raised and recessed portions of the guide groove as information tracks, thereby increasing the track density. In this case, each groove track and land track are formed approximately equal in width.
In the method employed in this recording medium, address information is provided between the groove track and land track so that one address section is shared between each adjacent groove track and land track pair. The address section recorded between the adjacent groove track and land track is called the “intermediate address”.
FIG. 11 is a diagram showing the format of the optical recording medium containing the intermediate addresses provided within zones. Reference numeral 1 is the groove track, 2 is the land track, 3 is the intermediate address, and 4 is the recording area.
Here, each intermediate address 3 is formed from a pair of address pits 3a and 3b shifted relative to each other by a half track pitch in the radial direction. In the figure, the address pits are shown as being rectangular in shape, though they are actually elliptical in shape. In an alternative embodiment, the address section may be formed, not from such a pair of address pit sequences, but from address pit sequences not shifted in the radial direction. In that case, the center of the address section is taken at the center position of the address pits in the radial direction.
A first feature of the intermediate address 3 of this embodiment is that one address is reproduced from both the groove track 1 and the land track 2, and that since there are two addresses on both sides of each track, high reliability can be provided during address demodulation.
A second feature is that the center position of the recording track can be accurately obtained from the reproduced signals of the address pits 3a and 3b located on the left and right sides. More specifically, by comparing the amplitudes of the signals reproduced from the address pits 3a and 3b, and by performing control so that the reproduced signals from the two addresses become equal in amplitude, tracking servo that follows the track center becomes possible. Because of these two features, the intermediate address 3 is used for land/groove recording.
However, when reproduced signals from a recording medium manufactured using the above-configured substrate were closely measured, a phenomenon was observed in which the relative positional relationship between the center of the guide groove and the center position of the intermediate address was displaced between the starting position (radially inward position) and the end position (radially outward position) of each of the plurality of zones arranged in the radial direction.
More specifically, this phenomenon means that the condition under which the amplitudes of the reproduced signals from the intermediate addresses become equal, relative to the center position of the tracking error signal, greatly varies in boundary portions of each zone.
If the center position of the intermediate address is expressed in terms of the positional displacement from the center position of the guide groove, a reproduced signal of the polarity that occurs when the entier intermediate address is shifted toward the inner diameter is obtained at the starting position of the zone while, at the center of the zone, it coincides with the center of the guide groove and, at the end position of the zone, a reproduced signal of the polarity that occurs when the intermediate address is shifted toward the outer diameter is obtained.
As a result, when tracking control is performed by obtaining the track center from the intermediate address, as described above, at the starting position of the zone as well as at the end position of the zone, the tracking servo operates in an off track condition, thus tracking a position displaced from the center of the guide groove.
If signal recording is performed in this condition, there arises the problem that not only does the quality of the signal itself degrade, but also the signal is prone to crosstalk from adjacent tracks, making stable recording and reproduction difficult.